Linear actuators, in particular spindle actuators, for hydrostatic master units are known from the prior art, in particular for an actuating device of a friction clutch of a motor vehicle. For example, hydrostatic master units are known in the form of coupling actuators for the hydraulic actuation of a friction clutch, which include, for this purpose, the master cylinder of a hydrostatic actuating system. In the case of a hydrostatic clutch actuator, a master cylinder is generally provided for accommodating a hydraulic fluid, wherein the accommodation volume can be changed via a master piston. The master piston is translationally moved in the master cylinder, thereby changing the accommodation volume of the master cylinder. In order to control the change in volume, a rotary (e.g. electric) drive may be utilized, and the rotational motion is converted into a translational motion of the master piston via a spindle drive. In one variant, a rotationally fixed spindle nut is translationally moved via a drive spindle. Reference is made in this regard to DE 10 2010 047 801 A1, for example. Yet another variant is known from WO 2011/050 766 A1, in which a master piston is situated in a master cylinder and is translationally fixed to a drive spindle, wherein the rotary drive is converted, therein via a planetary screw drive (PWG), this rotation into a translational motion of the drive spindle and, therefore, of the master piston.
In order to control or regulate the position of the linear actuator element (for example, a spindle nut or a drive spindle) of a linear actuator, for example the master piston, a translational absolute displacement sensor, for example, is utilized in the prior art for measuring the translational absolute displacement position directly at the linear actuator element. Such an absolute displacement sensor can unambiguously establish the position in which the linear actuator element is located, at any time during operation, via a direct (linear) measurement, without a need for a further reference for this purpose. The absolute displacement sensor is expensive and, in addition, is usually only spatially separated from the measurement electronics system and, therefore, can often be integrated into a linear actuator via additional plug connections, which require a great deal of installation space, and/or via more complex casings.
In addition, a configuration is known, in which all that is provided is a rotor position sensor (or angle sensor). In this case, the position of the linear actuator element is deduced at the level of the read-out electronics system or the evaluation electronics system, incrementally by counting every complete revolution of the rotary drive and indirectly by calculating the geometric relationship between the thread pitch and the relative translational displacement between the spindle nut and the drive spindle. In this configuration, it is necessary to provide at least one reference stop or one reference sensor, in the case of which a zero position is defined. This means that, in the case of a (memory) loss of the controller or in the case of an error during the counting of the full revolutions and, therefore, a loss of the reference to the reference stop, the reference stop or the reference sensor must be approached. A great deal of time is required in this case, in particular due to an additional plausibility check which is usually necessary. The plausibility check is necessary, for example, in order to be able to reliably differentiate a possible sluggishness or other stops from the reference stop. It is also disadvantageous in this case that the mechanical requirements on such an actuator are particularly high, because a high running quality and a high impact resistance for the reference stop are required.
In addition, systems which read out indirectly are known, in the case of which driven measurement shafts are utilized for the read-out. Such systems can be designed in such a way that a reference stop is not required, because assigned to every angular position is a unique measurement value, along the entire displacement path of the linear actuator element in some embodiments. With respect to linear actuators in particular, in which a central component (for example a translationally moved drive spindle) prevents a central measuring magnet rotating together with the rotor from being provided, at least one mechanically connected, off-axis measurement shaft has been previously used. As a result, inaccuracies in the mechanical transmission, for example via gears, are accepted and/or a high production precision and assembly precision are required.
Proceeding herefrom, the problem addressed by the prevent disclosure is that of at least partially overcoming the disadvantages known from the prior art. The features according to the disclosure become apparent from the independent claims, for which further embodiments are described in the dependent claims. The features of the claims can be combined in any technically meaningful way, wherein the explanations from the following description as well as features from the figures, which include further embodiments of the disclosure, can also be utilized therefor.